This invention relates to a system and a method for identifying the material composition of a sample by analysis of X-ray spectra emitted therefrom, and is most advantageous for identifying the material composition of defects found on semiconductor wafers.
During the fabrication of devices on semiconductor wafers, the wafers undergo periodic inspection for defects. When such defects are discovered, it is important to identify the root-cause for the defects in order to correct the problem and avoid introducing similar defects to other wafers. However, it is prohibitively expensive to shut down the production line while a prolonged analysis of the root-cause is being performed. Therefore, the more information is gathered about the defect in the shortest time, the faster the root cause can be identified and the proper corrective actions can be implemented. One piece of important data which can help identify the root-cause is the composition of the material of the defect. Many attempts have been made in the art to obtain such identification.
One of the common techniques used in the microelectronics industry for analyzing the composition of materials is Energy Dispersive X-ray Spectroscopy (widely referred to as EDX or EDS) analysis. EDX analysis is generally performed using a Scanning Electron Microscope (SEM). The sample to be analyzed is irradiated by a primary beam of electrons, which causes x-ray emission from the sample""s surface as the electrons of the atoms on and near the surface fall from their excited states to lower energy states. In general, the fundamental emissions, which are denoted as K, L and M-series are unique for each element. This provides a xe2x80x9cfinger printxe2x80x9d which enables identification of the elements present on or near the surface of the sample. Both qualitative and quantitative analysis can be performed. The intensity of the x-ray signal is determined by a number of factors, such as the primary beam""s energy, the detector""s angle, the film thickness, the surface roughness and the concentration of the elements within the sample. The latest technology can detect elements from Be to U with high accuracy.
Wavelength-dispersive spectrometers (WDS or XRF) and EDX are also widely used in plasma research and various thin film and surface analysis. Examples of various x-ray material analysis systems include: Philips PW1400 wavelength dispersive X-ray fluorescence spectrometer; Rigaku RIX-3000; Kevex energy-dispersive X-ray fluorescence spectrometer; Voyager by Noran and Link by Oxford. For further informative reading, the reader is referred to U.S. Pat. Nos. 5,659,172; 5,118,041; 5,065,020; 4,988,872; and 4,382,183, the teachings of which are incorporated herein by reference.
As noted above, EDX analysis is used in the semiconductor industry, among others, to analyze the composition of defects on the wafers. An SEM system having EDX capabilities is exemplified in FIG. 1. An electron source 100 is activated to emit electrons, which are then formed into a primary electron beam 110 by lenses 120 and 130. Deflection coils 140 are used to direct and/or scan the beam onto the sample 150. The generated secondary electrons (SE) and the back-scattered electrons (BSE) are sensed by the electron detector 165, the output of which is used to generate an SEM image of the sample. Additionally, when the EDX system is activated, x-rays emitted from the sample 150 are detected by sensor 160, the signal of which is amplified by an amplifier 170 and sent to processor 180 for processing. The processor is connected in a known manner to a user interface 185 and memory 190. The output of the processor is provided in the form of a plotted spectrum 195.
In examining a sample using the system exemplified in FIG. 1, the user directs the primary electron beam onto the detected defect and acquires the x-ray emission. The processor 180 then displays the spectrum 195 of the acquired X-ray emission, and the user analyzes the spectra peaks to obtain a list of the elements known to produce such peaks. However, this manual method is slow, cumbersome, and is affected by the fact that x-rays emitted from the defect include x-rays emitted from the background. Consequently, it is rendered hard, and sometimes impossible, to distinguish between the material of the defect and the material of the background, i.e., the wafer. This is particularly problematic in patterned wafers in which the top layer may include various elements which constitute the dielectric, metal lines, contact holes, etc.
In the prior art, it is attempted to overcome this difficulty by separately acquiring the x-ray spectra of the substrate and of the particle. In practice, the user has to manually point the primary beam to a selected location on the defect and a selected location on the background. The selection of the appropriate location is made by the user in reliance on his knowledge and experience. Then, the user qualitatively identifies spectra peaks that appear in both the background and the substrate, and decides based upon his experience whether to attribute each of such common element to the substrate. As can be seen, in addition to being slow, the results obtained from such a process can vary from operator to operator, depending on their knowledge and experience. Thus, there""s a need in the prior art for a system which automatically investigates the defect and the background spectra to correctly identify the material constituting the defect.
The present invention provides a system for automatic EDX analysis of defects, quantitatively taking into consideration x-ray signal attributable to the background. The system is particularly beneficial for analysis of defects on semiconductor wafers and, due to its automation, is suitable for in-line inspection of wafers in the fabrication plant.
One advantageous feature that enables the system to have a high throughput is termed xe2x80x9ctrace element analysis.xe2x80x9d As is known, basically two types of particle defects can be present on a wafer: one type is a leftover processing material, such as a particle left from etching, a photoresist residue, etc. The other type is xe2x80x9cforeignxe2x80x9d particles, i.e., particles introduced from external sources, such as the processing chamber""s walls, the chuck holding the wafer, vacuum and gas lines, etc. In production-line monitoring, it is very important to rapidly identify foreign particles since they indicate that a processing chamber is failing and requires repair or service. Accordingly, in the trace element analysis the system analyzes the x-ray spectra obtained and, if an element that under no circumstances should be present on a wafer is noted, such as iron for example, the system immediately issues an alarm that a foreign particle has been introduced. This helps focus the yield engineer to investigate problems relating to the equipment and not the process.
Another advantageous feature of the invention is its ability to automatically perform EDX analysis of defects, taking into consideration x-ray signal attributable to the background. Specifically, the system is capable of automatically identifying suitable locations for background and defect x-ray sampling. The system is also capable of effectively and quantitatively, rather than qualitatively, remove signals attributable to the background and not the defect.
The general steps of the inventive method include (not necessarily in that order):
1xe2x80x94detecting the defect;
2xe2x80x94analyzing the image of the defect and its surroundings;
3xe2x80x94determining the preferred point on the defect for the defect EDX spectrum acquisition, and acquiring the EDX spectrum of the defect;
4xe2x80x94analyzing the defect spectrum and performing the simple trace element analysis;
5xe2x80x94either determining the preferred point on the substrate for the substrate EDX spectrum acquisition and acquiring an x-ray spectra from the preferred point, or;
6xe2x80x94comparatively analyzing the defect and background spectra to yield a net defect spectrum identifying the composition of the defect material;
7xe2x80x94comparing the net spectrum to the spectra in a defect material spectrum library to identify the nature and source of the defect.